To meet the ever-increasing demand on communication capacity, optical transmission systems are moving towards high channel data rate (e.g., 100's-Gb/s/channel or Terabit/s/channel) and high spectral efficiency (SE). Digital coherent detection is powerful technique capable of fully recovering the complex field of a received signal, allowing the reception of high SE signals and the compensation of linear impairments including chromatic dispersion (CD) and polarization-mode dispersion (PMD) using digital filters. However, fiber nonlinearities impose a severe limitation on the transmission performance of coherent signals, especially high SE signals with large constellation size. It is desired to improve the signal tolerance to fiber nonlinear effects. Interleaved return-to-zero (IRZ) polarization-division multiplexing is a promising modulation technique to support high data-rate transmission with high signal tolerance to fiber nonlinear effects.
As is well known, an optical signal may have two orthogonal polarization states, each of which may have different properties. Sometimes such polarization states are intentionally introduced, such as in creating a polarization-multiplexed signal in which the two orthogonal polarization states of the optical carrier are arranged so that each carries different data in order to double the spectral efficiency. Such a polarization-multiplexed signal has two so-called “generic” polarization components, each of which carries a single data modulation. Note that by a generic polarization component it is generally intended the signal at the point at which the modulation of that polarization component is completed. It should be appreciated that each generic polarization component may initially, or otherwise, exist separate from the other generic polarization component with which it is later combined. It should also be appreciated that the phase of the generic need not be constant.
There are two conventional methods for generating IRZ-PDM signals. A first conventional method of implementing generation of an IRZ-PDM signal is shown in FIG. 1. In particular, FIG. 1 illustrates a first conventional implementation for generating IRZ-PDM signals that are modulated using quadrature phase-shift keying (QPSK). As shown, the example transmitter 100 arranges one pulse carver 102, one polarization-beam splitter (PBS) 104, one polarization-beam combiner (PBC) 106, and two I/Q modulators 108, 109. In addition, there is a fixed optical delay 110 between the two modulator paths to produce the example IRZ-PDM-QPSK signal. Other modulation formats such as quadrature amplitude modulation (QAM), binary phase-shift keying (BPSK) etc. may be utilized by the transmitter for the modulation scheme of the transmitted signal.
In further detail, output signal from a laser source 112 is provided as input to pulse carver 102 for production of corresponding return to zero (RZ) signal. The pulse carver also receives as input a clock C1. The clock signal has a frequency of 1/Ts where Ts is the modulation symbol period. For instance, the clock may be a 28-GHz clock such that the pulse carver produces a RZ pulse train at a repetition rate of 28 GHz.
The RZ signal from the pulse carver 102 is directed to PBS 104. The PBS splits the incident beam into two beams of differing linear polarization, with each of the beams provided to a respective I/Q modulator 108, 109. A first I/Q modulator 108 handles modulation of the in-phase (I1) and quadrature (Q1) components of a first signal intended to be transmitted (e.g., an x-polarization). A second I/Q modulator 109 handles modulation of the in-phase (I2) and quadrature (Q2) components of a second signal intended to be transmitted (e.g., a y-polarization). There is a fixed delay 100 between the two I/Q modulator paths that is equal a half symbol period, Ts/2. As illustrated in FIG. 1, for example, each of the polarizations may be a 56-Gb/s RZ-QPSK signal after modulation.
After delay of one of the polarizations (e.g., the second polarization, the y-polarization), the first and second polarization are combined by the PBC 106 to produce the resultant modulated IRZ-PDM signal. For example, as illustrated in FIG. 1, the resultant modulated signal may be a 112 Gb/s IRZ-PDM-QPSK signal.
A second conventional method of implementing generation of an IRZ-PDM signal is shown in FIG. 2. In particular, FIG. 2 illustrates a second conventional implementation of the generation IRZ-PDM signals that are modulated using QPSK. Other modulation formats such as QAM and BPSK etc. may be utilized by the transmitter for the modulation scheme of the signal to be transmitted. As shown, the example transmitter 200 arranges two pulse carvers 202, 204, one polarization-beam splitter (PBS) 206, one polarization-beam combiner (PBC) 210, and two I/Q modulators 208, 209. In addition, a necessary delay of one half a symbol period (i.e., ½Ts-delay) between the two modulator paths is realized by delaying the drive signals of the pulse carver and I/Q modulator of one modulator path with respect to the pulse carver and I/Q modulator of the other respective modulator path.
In further detail, an output signal from a laser source 212 is directed to PBS 206. The PBS splits the incident beam into two beams of differing linear polarization, with each of the beams provided as input to a respective pulse carver 202, 204 for production of a corresponding return to zero (RZ) signal. A first pulse carver (e.g., pulse carver 202) receives a first beam from the PBS and also receives as input a first clock C1. First clock signal C1 has a frequency of 1/Ts where Ts is the modulation symbol period. A second pulse carver (e.g., pulse carver 204) receives a second beam from the PBS and also receives as input a second clock C2. Second clock signal C2 has a frequency of 1/Ts where Ts is the modulation symbol period but is delayed by a half symbol period (i.e., ½Ts) with respect to clock signal C1. For instance, both clock signals may be a 28-GHz clock such that corresponding pulse carvers produce a 28-GHz RZ output signal for each path, one output signal delayed with respect to the other output signal.
The RZ signal from each pulse carver is provided to a corresponding I/Q modulator 208, 209. The first I/Q modulator 208 handles modulation of the in-phase (I1) and quadrature (Q1) components of a first signal intended to be transmitted (e.g., an x-polarization). The second I/Q modulator 209 handles modulation the in-phase (I2) and quadrature (Q2) components of a second signal intended to be transmitted (e.g., a y-polarization). The I2/Q2 components are also delayed by a half symbol period (i.e., ½Ts) with respect to the I1/Q1 components. Thus, drive signals of the second pulse carver 204 and the second I/Q modulator 209 are delayed (e.g., by a fixed delay, by an adjustable delay) with respect to those of the first pulse carver 202 and first I/Q modulator 208. For example, as illustrated in FIG. 2, the output of a respective I/Q modulator 208, 209 may be a 56-Gb/s RZ-QPSK signal for a respective polarization.
The first and second polarizations (e.g., the x-polarization and the y-polarization) output from the respective I/Q modulators are combined by PBC 210 to produce the result modulated IRZ-PDM signal. For example, as illustrated in FIG. 2, the resultant signal may be a 112 Gb/s IRZ-PDM-QPSK signal.